The present invention relates to a video signal processing circuit adapted in a video apparatus such as TV, VTR, video camera, etc., and particularly to a horizontal edge compensation circuit for greatly reducing a transition time of a luminance signal level and performing a horizontal edge compensation, to improve a sharpness of the reproduced image.
Generally, when the number of lines per frame and the number of frames per second have been fixed, the horizontal resolution is largely a function of a system bandwidth during reproduction of the recorded video signal. The frequency spectrum of a standard composite video signal for use in a NTSC color television is as shown in FIG. 1, where the bandwidth of the luminance signal Y is 4.2 MHz, and in the color difference signal channels, a 1.3 MHz bandwidth for the I channel and a 500 KHz bandwidth for the Q channels are arranged centering around a color subcarrier frequency fsc of 3.58 MHz. More particularly, the recent NTSC helical scan video recorders have resulted in the appearance of low luminance bandwidth (2 MHz) and chroma bandwidth (350 KHz) pictures, due to their high portability and low cost. These low bandwidth composite video signals make it difficult to obtain a high-resolution picture. If the luminance and chrominance signal bandwidths were wider, a sharper picture would be possible. However, in this case, the transmitting and receiving systems should be modified, which would increase the production costs of the transmitter and receiver. And so, studies on the horizontal edge compensation have been continued in order to enhance the sharpness of pictures without any change to the bandwidths of the standard luminance and chrominance signals. Examples of prior arts relevant to the technological field of the present invention are described in U.S. Pat. No. 4,030,121 to Faroudja, U.S. Pat. No. 4,414,564 to Hitchcock, U.S. Pat. No. 4,263,616 to Lee, U.S. Pat. No. 4,853,783 to Ozaki.
FIG. 2 shows a conventional horizontal edge compensation circuit. The circuit of FIG. 2 comprises a first path including a delay circuit 10 for delaying an input luminance signal for a certain period, a second path including a band-pass filter 20 which is tuned to a frequency to compensate the horizontal edge, and an adder 70 for summing signals supplied from the first and second paths. The second path includes band-pass filter 20, coring unit 21, gain control unit 22, and limiter 60.
FIG. 3 illustrates the frequency response of the band-pass filter 20 which is tuned to 3.58 MHz.
FIGS. 4A through 4D illustrate waveforms showing operational characteristics of the respective components of the horizontal edge compensation circuit of FIG. 2 including the above band-pass filter 20. The small circles 5 represent sampling points with the frequency of the sampling clock at 4 fsc (4.times.3.58=14.3 MHz).
If luminance signal Y as shown in FIG. 4A is fed to the horizontal edge compensation circuit of FIG. 2, the band-pass filter 20 differentiates the luminance signal Y as shown in FIG. 4B, and then, outputs a frequency component tuned to 3.58 MHz. The band-pass filter 20 differentiates the above differentiated signal again, and then outputs the signal as shown in FIG. 4C. The coring unit 21 removes the noises having small amplitude impulse components or noise spikes from the edge-compensating component output from the band-pass filter 20. The gain control unit 22 controls the gain of the noise-removed horizontal edge compensating component. The gain control unit 22 controls the size of the horizontal edge compensating signal in order to compensate the horizontal edge of a primary signal as shown in FIG. 5. The limiter 60 limits the horizontal edge compensating signal generated from the gain control unit 22 to a certain size, and supplies the signal to the adder 70. The adder 70 adds the luminance signal supplied via the first path including the delay circuit 10 to the horizontal edge compensation signal supplied via the second path, and outputs luminance signal Y.sub.1 in which the horizontal edges are compensated as shown in FIG. 4D. At this time, the horizontal edge compensating signal of FIG. 4C is actually subtracted from the primary signal.
The signal whose horizontal edge is compensated as shown in FIG. 5 enhances the sharpness of the displayed video signal by further increasing the level difference in the transition interval of the video signal. The effects of the conventional horizontal edge compensation circuit, however, are extremely limited because the circuit compensates the edge of a single extracted frequency component of limited bandwidth.
FIG. 6 illustrates another embodiment of the conventional horizontal edge compensation circuit.
The horizontal edge compensation circuit in FIG. 6 which is an improvement over the circuit in FIG. 2, comprises a delay circuit 10 for delaying the input luminance signal Y for a certain period, band-pass filters 20, 30 and 40 which are each supplied with luminance signal Y, coring units 21, 31 and 41 which are coupled to the output of each band-pass filter, gain control units 22, 32 and 42 which are coupled to the output of each coring unit, a mixer 50 for mixing the output signal from each gain control unit, and a limiter 60 for limiting the magnitude of the output signal from the mixer 50, an adder 70 for adding the luminance signal supplied from the delay circuit 10 to the horizontal edge compensation signal supplied from the limiter 60 to provide the luminance signal Y.sub.2 whose horizontal edge is compensated.
The basic operation of the horizontal edge compensation circuit in FIG. 6 is the same as that of the circuit in FIG. 2. However, the horizontal edge compensation circuit of FIG. 6 obtains a horizontal edge compensation signal over a wider frequency bandwidth than that of the circuit of FIG. 2. For instance, the band-pass filters 20, 30 and 40 extract the component tuned to the frequencies of f1=2.3 MHz, f2=3.58 MHz and f3=4.58 MHz as shown in FIG. 7, respectively. Then, the mixer 50 does not emphasize the particular horizontal edge compensation component by simply adding the respective horizontal edge compensation components extracted at the different frequency bandwidths each other according to the characteristics of the band-pass filters 20, 30 and 40, but it emphasizes or suppresses the particular edge compensation component by adding or subtracting the three horizontal edge compensation component to or from one another. The characteristic of the mixer 50 for mixing the three different horizontal edge compensation components appropriately in order to emphasize a particular component is shown in FIG. 8.
FIG. 8 in which the axis of ordinate represents gains and the axis of abscissa represents frequencies illustrates the horizontal edge compensation signals having three different frequencies f1, f2 and f3. Such different horizontal edge compensation signals are added or subtracted to or from one another to obtain various horizontal compensation signals.
The horizontal edge compensation signal is added to the primary signal which passed through the delay circuit 10 of FIG. 6, thereby obtaining a sharper horizontal edge-compensated luminance signal Y.sub.2. However, since the aforementioned two conventional horizontal edge compensation circuits extract horizontal edge compensation components from particular frequencies to add the components, the circuits have disadvantages in that, when the level of video signal is changed, a trailing phenomenon that the falling length tails away occurs because an overshot frequency component is converted to higher harmonics of the particular frequencies and the transition time of a video signal cannot be contracted.
In order to solve these problems, still another conventional horizontal edge compensation circuit is as shown in FIG. 9.
The horizontal edge compensation circuit in FIG. 9 comprises delay circuit 85 for delaying an input luminance signal Y for a certain period, a first differentiator 80 for differentiating the luminance signal Y, a second differentiator 81 for again differentiating the output of the first differentiator, a limiter 83 for limiting the size of the second differentiated signal, an absolute value (ABS) circuit 82 for obtaining the absolute value of the first differentiated signal, a multiplier 84 for multiplying the absolute value of the first differentiated signal by the limited second differentiated signal, and an adder 86 for summing the outputs of multiplier 84 and delay circuit 85, to output a horizontal edge-compensated luminance signal Y.sub.3.
If the luminance signal Y as shown in FIG. 10A is input to the horizontal edge compensation circuit of FIG. 9, the luminance signal Y is differentiated by the first differentiator 80 to provide the first differentiated signal as shown in FIG. 10B. The first differentiated signal is supplied to the second differentiator 81 and simultaneously is passed through the absolute value circuit 82 coupled in parallel to the second differentiator 81. The signal as shown in FIG. 10C which is obtained by differentiating again the first differentiated signal in the second differentiator 81 and then passing through the limiter 83, is supplied to the multiplier 84. The multiplier 84 multiplies the absolute value of the first differentiated signal supplied through the absolute value circuit 82 by the signal output from the limiter 83, resulting in obtaining the horizontal edge compensation signal as shown in FIG. 10D.
The horizontal edge compensation signal obtained by multiplying the first differentiated signal by the second differentiated signal becomes harmonics higher than 3.58 MHz horizontal edge compensation signal. In the adder, the horizontal edge compensation signal is added to the primary signal which has been delayed for a certain time in the delay circuit 85, to become a horizontal edge-compensated luminance signal Y.sub.3 as shown in FIG. 10E. At this time, the horizontal edge compensation signal of FIG. 10D is actually subtracted from the primary signal.
As described above, the conventional horizontal edge compensation circuit shown in FIG. 9 improves the sharpness of a picture by reducing the trailing of the horizontal edge compensation signal and also the transition time of the level of a video signal. But, since the horizontal edge compensation circuit in FIG. 9 adds a second higher harmonics to the horizontal edge compensation component, the circuit has a unnecessarily long trailing and roll-off that the picture becomes dim even if the transition time of the level of the video signal is short.